The invention relates to an optically readable information disk which comprises a substrate plate having at least on one side a radiation-cured lacquer layer in which an information track of information regions situated at a higher and a lower level is provided and is covered with a reflecting layer.
Such a disk is known from the published Netherlands Patent Application No. 76 11 395 in the name of Applicants.
Applicants have found that the known information therefrom have a considerably worse signal-to-noise ratio, abbreviation SNR, than the master plate from which the disks are derived. The master plate is a flat glass plate which is covered on one side with a photoresist layer in which the information track has been provided by exposure to laser light succeeded by photochemical development. First metal copies, the so-called father disks, are made from the master plate electrochemically and by electro-deposition, from which father plates, subsequently second metal copies, also called mother disks, are made by electro-deposition and therefrom third and subsequent metal copies are manufactured. The last copies of the "family" are termed dies.
The above-mentioned optically readable information disks are manufactured by means of the dies in which the surface of the die in which the information track is present is provided with a thin layer of a lacquer which is curable by radiation, for example ultraviolet light, the substrate plate is then provided thereon, the assembly is exposed via the substrate plate and the resulting disk comprising the substrate plate and the cured lacquer layer connected thereto in which the information track is copied, is finally removed from the die. The cured lacquer layer is provided with a reflecting layer, for example, a vapour-deposited layer of aluminum or silver, on the side of the information track.
The information regions present in the information track have very small dimensions. The longitudinal dimensions vary in accordance with the stored information from approximately 0.3-3 .mu.m. The difference in height between the regions mutually corresponds to a quarter wavelength (or a multiple thereof) of the laser light with which the disk is read optically and amounts to 1-2 .mu.m. The disk is read in reflection via the substrate plate in which the detection of the information regions is based on phase differences occurring between the forward and the reflected laser light beam.
In the steps leading from the master to die and the subsequent production of the optically readable disk starting with the provision of a light-curable lacquer on the die and up to the vapour deposition of a reflecting layer, all kinds of errors and deviations may occur as a result of the quality of the finished disk is diminished, in particular a deterioration in the signal-to-noise ratio with respect to that of the master.